To enable a number of operational amplifiers to be used in parallel for driving a common load, it has hitherto been necessary to connect ballast resistors between the outputs of each amplifier and the load, to equalize the currents from each amplifier. An example of such an arrangement is shown in FIG. 1. FIG. 1 shows an amplifier bridge system wherein a load 23 is powered from two pairs of operational amplifiers 1, 2 and 3, 4. Each operational amplifier 1-4 has a ballast resistor Rb placed between the output and the load 23.
The inclusion of such resistors Rb is made necessary by the large cross-conduction currents brought about by the low output impedance of the operational amplifiers 1-4, together with the presence of gain offset or mismatch, which currents would be circulated among the outputs of the operational amplifiers 1-4. Use of ballast resistors Rb results in less power being delivered to the load 23 and more power being dissipated, as well as increasing the cost and size of the system. Low resistance, high-precision power resistors Rb, usually resistance wires, must be employed. These resistors Rb tend to be bulky and significantly expensive.